Article

Isoform-specific functions of protein kinase C: The platelet paradigm

Department of Physiology and Pharmacology, University of Bristol, School of Medical Sciences, University Walk, Bristol BS8 1TD, U.K.
Biochemical Society Transactions (Impact Factor: 3.24). 12/2007; 35(Pt 5):1005-8. DOI: 10.1042/BST0351005
Source: PubMed

ABSTRACT Platelets are central to haemostasis and thrombosis. Many key steps in platelet activation and aggregation are regulated by members of the PKC (protein kinase C) family. Multiple isoforms of PKC are expressed in platelets, and evidence is emerging that different isoforms play distinct roles in the platelet activation process. This may, in part, be regulated by isoform-specific interactions between PKC family members and other intracellular signalling molecules, such as tyrosine kinases, or the actin cytoskeleton regulator, VASP (vasodilator-stimulated phosphoprotein). The contributions of individual PKC isoforms can be addressed directly in platelets from knockout mouse models, which are providing key insights into the physiological function of PKC isoform diversity and can be a valuable complimentary approach to more commonly used pharmacological analyses. Using knockout mouse models, recent reports have demonstrated the importance of PKCbeta and PKCtheta in integrin-dependent platelet spreading, and also a novel role for PKCdelta in regulating filopodial formation, highlighting the utility of such models to investigate the functions of specific PKC isoforms in a physiological process that is significant to our understanding of cardiovascular disease.

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    • "Platelets therefore play a pivotal role in the thrombus formation, as well as in the plaque development [1] from the very beginning of atherosclerotic disease. A variety of platelet functions have been associated with PKC activity [2]. PKC activity, in synergy with Ca2+, regulates the secretion of dense and α-granules following platelet stimulation with phospholipase C-stimulating agonists, like collagen and thrombin [3]–[5]. "
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    ABSTRACT: Platelets play crucial roles in the pathophysiology of thrombosis and myocardial infarction. Protein kinase C ε (PKCε) is virtually absent in human platelets and its expression is precisely regulated during human megakaryocytic differentiation. On the basis of what is known on the role of platelet PKCε in other species, we hypothesized that platelets from myocardial infarction patients might ectopically express PKCε with a pathophysiological role in the disease. We therefore studied platelet PKCε expression from 24 patients with myocardial infarction, 24 patients with stable coronary artery disease and 24 healthy subjects. Indeed, platelets from myocardial infarction patients expressed PKCε with a significant frequency as compared to both stable coronary artery disease and healthy subjects. PKCε returned negative during patient follow-up. The forced expression of PKCε in normal donor platelets significantly increased their response to adenosine diphosphate-induced activation and adhesion to subendothelial collagen. Our data suggest that platelet generations produced before the acute event retain PKCε-mRNA that is not down-regulated during terminal megakaryocyte differentiation. Results are discussed in the perspective of peri-infarctual megakaryocytopoiesis as a critical component of myocardial infarction pathophysiology.
    PLoS ONE 10/2012; 7(10):e46409. DOI:10.1371/journal.pone.0046409 · 3.23 Impact Factor
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    • "The role of individual isoforms in platelet activation has been investigated using isoform-specific inhibitors and mice deficient in single isoforms [4–7,9–19]. This has led to the conclusion that the classical isoforms play positive roles in platelet activation, with PKCα playing the predominant role, supported by PKCβ, while the novel isoforms play minor or inhibitory roles [10]. "
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    ABSTRACT: In comparison to the classical isoforms of protein kinase C (PKC), the novel isoforms are thought to play minor or inhibitory roles in the regulation of platelet activation and thrombosis. To measure the levels of PKCθ and PKCε and to investigate the phenotype of mice deficient in both novel PKC isoforms. Tail bleeding and platelet activation assays were monitored in mice and platelets from mice deficient in both PKCθ and PKCε. PKCε plays a minor role in supporting aggregation and secretion following stimulation of the collagen receptor GPVI in mouse platelets but has no apparent role in spreading on fibrinogen. PKCθ, in contrast, plays a minor role in supporting adhesion and filopodial generation on fibrinogen but has no apparent role in aggregation and secretion induced by GPVI despite being expressed at over 10 times the level of PKCε. Platelets deficient in both novel isoforms have a similar pattern of aggregation downstream of GPVI and spreading on fibrinogen as the single null mutants. Strikingly, a marked reduction in aggregation on collagen under arteriolar shear conditions is observed in blood from the double but not single-deficient mice along with a significant increase in tail bleeding. These results reveal a greater than additive role for PKCθ and PKCε in supporting platelet activation under shear conditions and demonstrate that, in combination, the two novel PKCs support platelet activation.
    Journal of Thrombosis and Haemostasis 07/2012; 10(9):1887-94. DOI:10.1111/j.1538-7836.2012.04857.x · 5.55 Impact Factor
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    • "The protein kinase C (PKC) family critically regulates platelet activation. Many platelet functional responses, including secretion and aggregation are reduced or abolished by broad-spectrum PKC inhibitors and enhanced by PKC activators [1], suggesting a positive role for the PKC family in general in platelet activation. However, calcium responses are clearly negatively regulated by PKC isoforms [2], and we have shown by pharmacological and genetic approaches that PKCδ is a negative regulator of platelet aggregation by modulating actin dynamics through VASP [3], [4]. "
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    ABSTRACT: PKCtheta is a novel protein kinase C isozyme, predominately expressed in T cells and platelets. PKCtheta(-/-) T cells exhibit reduced activation and PKCtheta(-/-) mice are resistant to autoimmune disease, making PKCtheta an attractive therapeutic target for immune modulation. Collagen is a major agonist for platelets, operating through an immunoreceptor-like signalling pathway from its receptor GPVI. Although it has recently been shown that PKCtheta positively regulates outside-in signalling through integrin alpha(IIb)beta(3) in platelets, the role of PKCtheta in GPVI-dependent signalling and functional activation of platelets has not been assessed. In the present study we assessed static adhesion, cell spreading, granule secretion, integrin alpha(IIb)beta(3) activation and platelet aggregation in washed mouse platelets lacking PKCtheta. Thrombus formation on a collagen-coated surface was assessed in vitro under flow. PKCtheta(-/-) platelets exhibited reduced static adhesion and filopodia generation on fibrinogen, suggesting that PKCtheta positively regulates outside-in signalling, in agreement with a previous report. In contrast, PKCtheta(-/-) platelets also exhibited markedly enhanced GPVI-dependent alpha-granule secretion, although dense granule secretion was unaffected, suggesting that PKCtheta differentially regulates these two granules. Inside-out regulation of alpha(IIb)beta(3) activation was also enhanced downstream of GPVI stimulation. Although this did not result in increased aggregation, importantly thrombus formation on collagen under high shear (1000 s(-1)) was enhanced. These data suggest that PKCtheta is an important negative regulator of thrombus formation on collagen, potentially mediated by alpha-granule secretion and alpha(IIb)beta(3) activation. PKCtheta therefore may act to restrict thrombus growth, a finding that has important implications for the development and safe clinical use of PKCtheta inhibitors.
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